1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method for driving a liquid crystal of a thin film transistor liquid crystal display (TFT-LCD). Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing a bend state of the liquid crystal from restoring to a splay state even when an applied data voltage is lower than a transition voltage.
2. Discussion of the Related Art
In a liquid crystal display of the background art, as shown in FIG. 1, glass substrates 11 and 12 are facing into each other and separated by a spacer (not shown), thereby providing a space for a cell gap. A liquid crystal 13 is injected into the space and sealed by a sealing member 14.
Further, a plurality of pixel electrodes 15 are formed on the inner surface of the glass substrate 11. A thin film transistor 16 serving as a switching element is formed at each pixel electrode 15. An orientation film (not shown) is formed on the pixel electrodes 15. A drain electrode of the thin film transistor will be connected to the pixel electrode 15. In the meantime, a transparent common electrode 17, facing into the pixel electrodes 15, is formed at the inner surface of the transparent substrate 12, and another orientation film is formed on the common electrode 17.
FIG. 2 is a schematic view of a liquid crystal display implemented with a drive circuit in the background art. As sown in FIG. 2, the liquid crystal display includes a liquid crystal panel 20, a scan drive circuit 21, and a signal drive circuit 22 for driving the liquid crystal panel.
More specifically, the liquid crystal panel 20 includes a plurality of scanning lines 23 and a plurality of signal lines 24 intersect each other on a substrate in a matrix form. At the intersecting portion, a thin film transistor 16 and a pixel are disposed therein.
The scan drive circuit 21 sequentially applies a scanning signal, such as an ON signal to the gate of the thin film transistor and the scanning line. The signal drive circuit 22 applies an image signal to the signal line, so that the image signal can be transferred to the pixel through the thin film transistor 16 driven by the scanning signal.
The liquid crystal display is driven on the basis of the following principle. When the scan drive circuit 21 sequentially applies the scanning signal to the scanning line 23 of the liquid crystal panel 20, all of the thin film transistors 16 connected to the scanning line receiving the scanning signal are on. The signal applied to the signal line 24 of the liquid crystal panel 20 is transferred to the pixel through the source and the drain of the activated thin film transistor 16.
According to the above-mentioned driving principle, a pulse signal is sequentially applied to all gate electrodes. Thus, a signal voltage is applied to the corresponding source electrode, so that all pixels of the liquid crystal panel can be driven. In this way, an image of one frame is displayed, and images of other frames are then sequentially displayed, so that moving images can be displayed.
In displaying the image in the above-described way, a color image including enormous information cannot be displayed by driving only black and white states. Therefore, in performing a contrast display, there are several intermediate states between the black and the white states.
In a black and white liquid crystal display, when an intermediate voltage between the black and the white states is applied, an intermediate state such as the gray state displays information. In order to obtain an intermediate value of the voltage, the amplitude of the voltage pulse or the intensity of the voltage applied to the liquid crystal is adjusted.
In a color liquid crystal display, a displayed color is determined according to a degree of the contrast display. When a drive integrated circuit (IC) of six bits is used, sixty-four contrasts can be generated. In audio/video equipment or a monitor, which requires full colors, 256 contrasts can achieve more than 16,000,000 colors. As described above, since a liquid crystal display is equipment that displays information on a screen by adjusting a voltage applied to the liquid crystal, contrast is adjusted by changing a transmitting degree of light depending on the applied voltage.
FIGS. 3A to 3C are schematic views for describing a state transition of a liquid crystal 13 according to the applied voltages at a black state, a white state, and a splay state, respectively. In general, in a construction where the liquid crystal is oriented in parallel, a bend alignment is stable when the applied voltage is greater than a threshold voltage Vtr for a state transition. Conversely, a splay alignment is stable when the applied voltage is below the threshold voltage.
Therefore, when a state where the liquid crystal driving voltage V is lower than the threshold voltage Vtr, the bend alignment is unstable. This is because a state transition into the splay state occurs for several seconds or several minutes, as shown in FIG. 3C.
An electrically controlled birefringence (ECB) mode has been proposed to obtain a wide viewing angle in a liquid crystal display. In the ECB mode, when a voltage is applied to a liquid crystal cell, an arrangement of liquid crystal molecules changes due to a dielectric anisotropy of the liquid crystal. Thus, a birefringence index of the liquid crystal cell changes a light transmission factor.
Among the ECB mode, an optically compensated birefringence (OCB) mode, or a π-cell mode is widely adopted to utilize the characteristic of the bend alignment state.
In the OCB mode, molecules of the liquid crystal are arranged to be the splay alignment in the initial state where a voltage is not applied and the liquid crystal located at the upper and the lower substrates are oriented with the same direction, as shown in FIG. 3C.
When a voltage greater than the transition voltage Vtr is applied to the liquid crystal panel, the liquid crystal experiences a transition from the splay alignment to the bend alignment as shown in FIG. 3B. In general, in the OCB mode, the applied voltage must be always maintained higher than the transition voltage Vtr so as to stabilize the bend alignment. Further, by controlling a driving voltage over the transition voltage Vtr, a distribution of the molecules of liquid crystal is adjusted, thereby controlling a transmission factor.
In the OCB mode, since asymmetric alignments in upward and downward directions and left and right directions are eliminated through the bend alignment as shown in FIG. 3B, the characteristic of the viewing angle is much improved. Also, a wide viewing angle is achieved in comparison with the conventional type liquid crystal cell. However, even in the OCB mode having the above-mentioned bend alignment, an additional voltage for forming the bend alignment is necessary. Moreover, as described above, there are several problems such as instability of the bend alignment yet to be resolved.
Further, since the OCB mode utilizes the characteristic of the bend alignment, an operation range is limited to the applied voltage over the transition voltage Vtr from the initial splay state to the bend state. Thus, a maximum value Vmax of the driving voltage is too large to obtain a desired brightness or a desired contrast ratio (C/R). A contrast ratio is an indicator for estimating vividness of an image on the screen. The larger in a difference of the brightness is, the more vivid the image is. As a result, it is difficult to select a drive circuit or a drive IC. Moreover, a high pre-tilt and a high cell gap are inevitable in implementing the device.